I'd draw a distinction between "motives" and "ulterior motives" intended to decieve or mislead. Both types of motives occur.

As a quick segmentation analysis, we could group commentators on oil issues roughly into political groups, corporate groups, and intellectual groups. The categories are not mutually exclusive. Other groupings do, of couse, make sense.

Political bodies include the IEA, EIA, USGS, and OPEC. They often have to support a homogenized point of view that is politically expedient. Many inside the groups may not really believe what they say. Consider the outrageously high USGS estimates, but the fact that they also publicize Les Magoon's peak oil viewpoint. I feel a lot of these groups are either disingenuous or have clear ulterior motives.

Corporate groups are the oil companies and other associated producers and users. A very few choose to speak openly (Chevron's "Will you join us" campaign). Some speak in code (Exxon Mobil's graph showing non-OPEC supply flat within the decade). Most don't speak, for good reason. Saying that your industry has no long-term future in its current form is bad for stock prices, and could even lead to lawsuits. I think these corporates will tell the truth when they believe it's good for business, and be discreet and quiet when they think the truth causes more trouble than it is worth. I don't think there is much outright lying here, perhaps because of liability issues as much as anything else.

Intellectual groups are those people studying the issue from a more outside perspective. I suspect most of these people are sincere and motivated by the truth as they see it. There are diverse opinions in this group: I'd put the Lynch & Yergin cornucopian / economic camp in this groups; as well as Hubbertarians/ depletionists like Deffeyes, Campbell, Simmons; environment policy people like Heinberg; social critics like Kunstler; and the more extreme post-industrial and dieoff groups. Some have clear agendas, such as pushing environmental or climate change policies, or debunking peak oil. But that doesn't mean they are lying--it's how they see the truth.

Dave said in an earlier post that peak oil is not a matter of belief, but I disagree with that. I am a fervent peak oil believer, but I can't (yet) prove my belief to a smart skeptic. The data is simply too bad, too incomplete, too full of holes. Ultimately, we don't know people's motives, but we can judge their words and actions. I'd assume sincerity, even from those I vehemently disagree with, unless we have clear evidence to the contrary.